The present invention relates to flame-retardant acrylic fibers and, more particularly, to acrylic fibers containing vinylidene chloride polymers.
Acrylic fibers, i.e., those containing at least 85 weight percent acrylonitrile, are well known in the art and widely used in commercial textile manufacture due to their excellent physical properties. However, such fibers have the distinct disadvantage that they ignite rather easily upon exposure to flames or high temperatures. Attempts to remedy this deficiency have been many and varied. For example, copolymerization of acrylonitrile with a halogen-containing monomer such as vinylidene chloride has resulted in fibers having considerably improved flame resistance. Typically, however, such copolymer fibers (generally classed as modacrylic fibers) also exhibit reduced sticking temperature and high shrinkage at elevated temperatures, such as upon exposure to boiling water or steam.
In order to provide a better balance between flame-retardancy and other desirable physical properties, attempts have also been made to blend various copolymers of acrylonitrile. For example, U.S. Pat. No. 3,963,790 is directed to a flame-resistant fiber wherein at least 50 weight percent of the fiber is a copolymer of about 79.5 to about 58 weight percent of acrylonitrile, 20 to 40 weight percent vinylidene chloride and 0.5 to 2 weight percent of a comonomer which enhances dyeability and 1 to 50 weight percent of the fiber is a second copolymer of 20 to 50 weight percent of acrylonitrile and 50 to 80 weight percent of vinylidene chloride. Inasmuch as such fibers consist of no more than a blend of two modacrylic fiber-forming polymers, it is inherent that they cannot be expected to exhibit the excellent properties of acrylic fibers.
Other attempts at providing flame-retardant acrylic fibers have involved the use of a variety of extraneous additives, such as antimony oxides, halogenated paraffins, halogenated hydrocarbons, and low molecular weight phosphate esters. It could be expected that only moderate decrease in beneficial physical properties would result if a minor amount of such an additive were dispersed throughout the continuous phase of an acrylic fiber structure. However, it has been the typical experience that effective utilization of these additives has undesirably required their presence in rather high concentrations.
It is, therefore, an object of the present invention to provide a flame-retardant acrylic fiber by utilizing an additive whose presence would only moderately affect the desirable inherent physical properties, such as high sticking temperature and low shrinkage, of an unmodified acrylic fiber. It is a further object to provide a novel vinylidene chloride polymer which is particularly suited for use as such an additive.